hw/vfio: cpr-iommufd: Fix wrong usage of migrate_add_blocker_modes

The return value of API is 0 for success and negative error code for
failure. We'll check if the return value equals to 0.
Also, the MIG_MODE should be CPR_TRANSFER and CPR_EXEC instead
of 2 same bits.

The API usage is aligned with 'hw/vfio/cpr-legacy.c' after these 2
changes.

Fixes: 3ca0a0ab05c ("migration: Use bitset of MigMode instead of variable arguments")
Signed-off-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260121063418.2001326-1-jim.shu@sifive.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>